USB Power Pack as Power Supply

So I bet you have seen these USB Battery Packs that recharge phones and the like. I thought I should be able to use one to power a USB powered device other than just charge a phone. I got a USB to 3.5 mm cable and tried to run a digital picture frame and a wireless camera. I tested voltage on the 3.5 mm jack and see ~5.04 VDC with proper polarity (outside barrel negative, inside positive). The power pack has two USB sockets, one labeled 1A and the other 2A. Neither of the two devices tested with the setup showed any signs of life when connected and both had a documented current draw under 1.5 A. I tried both the 1A and 2A sockets on the power pack. Everything works fine with the appropriate 110 VAC adapter. What am I missing here?

A lot of USB chargers look at the two data pins in the USB connector before powering up (especially Apple stuff). I'll bet that is the missing link.

OTOH, I get old retired cell phone chargers at the thrift shop and use them as 5V power supplies for small projects all the time.

It's often a couple of resistors across the data pins. Should be examples on Google.

Take a look at this thread from when I had the same problem.
Ipod nano charging problem